1. Field of the Invention
The present invention relates to a radio or wireless tag reader/writer operable to read information from and write information on a radio or wireless tag which is capable of information writing and reading by radio or wireless communication.
2. Description of the Related Art
There is known an RFID (Radio Frequency Identification) system wherein an RFID tag reader/writer or a read-only interrogator is operable in a non-contact fashion to write information on and read information from small-sized RFID tags (transponders) which store desired information. This RFID system permits the information stored in the RFID tag, to be read by radio communication even where the RFID tag is stained or located at an invisible position. For this reason, the use of the RFID system is expected in various fields such as commodity administration and inspection. Patent document 1 describes an example of an inspection system using RFID tags.
Patent document 2 describes a known writer (writing device) arranged to write information on circuit elements of the above-described RFID tags. In this conventional writer, a tape of paper on which rectangular labels are bonded at a predetermined spacing interval is successively fed from a roll in a tape supply portion disposed on one side of a housing, and desired information generated by a module within the housing is successively written on an IC circuit portion of the circuit element of each of the RFID tags bonded on the respective labels of the tape, through a reading/writing antenna which is disposed along a feeding path of the tape and which transmits the information to the circuit element. The thus written labels carrying the RFID tags are eventually delivered from the other side of the housing.
Patent document 3 describes another type of known writer (writing device) arranged to write information on the circuit elements of the above-described RFID tags. In this conventional writer, desired information generated by the writing device is transmitted to the circuit element of the RFID tag bonded on each of the rectangular labels bonded on a tape of paper at a predetermined spacing interval while the tape of paper is fed along a feeding path, so that the information is written on the IC circuit portion of the circuit element of each RFID tag.                Patent document 1: JP-2002-308437A        Patent document 2: JP-2002-2026A (paragraphs 0002-0009 and FIGS. 1-4)        Patent document 3: JP-2002-230499A (paragraphs 0012-0021 and FIGS. 1 and 2)        
For writing and reading information on and from the RFID tag, the information must be transmitted to and from the RFID tag by radio communication. The conventional radio communication for this purpose uses a frequency band of 125 KHz which has been long used, and a frequency band of 13.56 MHz which has been most commonly used. However, the use of these frequency bands suffers from a short distance of the radio communication, and unfavorably limits the utility of the RFID tags. In view of this drawback, it is considered that a main stream in the future is the use of a radio-frequency such as UHF bands (e.g., a band of 830-930 MHz and a band of 2.45 GHz) which have been widely used in the European countries and the United States. On the other hand, the use of the radio-frequency which permits a long distance of the radio communication requires each selected one of the RFID tags to be shielded or identified, so that the information is written on only the selected or desired RFID tag by the radio communication. There exists a similar drawback for reading the information from each selected RFID tag. To solve these drawbacks, it is considered to reduce the output of the radio reader/writer. However, this solution still has a risk of failure to assure normal radio communication, due to a possible interference between the selected RFID tags and the non-selected RFID tags, depending upon the specific configuration of the RFID tags.
It is found that where the antenna of the RFID tag and the antenna of the RFID tag reader/writer are located as close as possible to each other without a contact of those two antennas, there arise variations of the resonance frequencies of the two antennas due to their mutual coupling. As a result of extensive and continuous research by the present inventors with respect to that interaction of the two antennas, the inventors have discovered an RFID tag reader/writer which permits a high degree of stability of radio communication with only the selected or desired RFID tags.